**LBO Crystal**

Lithium Triborate (LiB3O5) LBO is an excellent nonlinear optical crystal. It has a wide transparency range, moderately high nonlinear coupling, high damage threshold and desirable chemical and mechanical properties. This crystal is often used for second harmonic generation (SHG, also known as frequency doubling), for example of Nd:YAG lasers (1064 nm → 532 nm). LBO can be both critically and non-critically phase-matched. In the latter case the crystal has to be heated or cooled depending on the wavelength.

**Features of LBO**

• Broad transparency range from 160nm to 2600nm;

• High optical homogeneity (δn≈10-6/cm) and being free of inclusion;

• Relatively large effective SHG coefficient (about three times that of KDP);

• High damage threshold;

• Wide acceptance angle and small walk-off;

• Type I and type II non-critical phase matching (NCPM) in a wide wavelength range;

• Spectral NCPM near 1300nm.

**Photonchina LBO Chemical and Structural Properties**

Crystal Structure | Orthorhombic, Space group Pna2_{1}, Point group mm2 |

Lattice Parameter | a=8.4473Å,b=7.3788Å,c=5.1395Å,Z=2 |

Melting Point | About 834℃ |

Mohs Hardness | 6 |

Density | 2.47g/cm^{3} |

Thermal Expansion Coeficients | α_{x}=10.8×10^{-5}/K, α_{y}=-8.8×10^{-5}/K,α_{z}=3.4×10^{-5}/K |

Thermal Conductivity Coefficients | 3.5W/m/K |

**Photonchina LBO Optical and Nonlinear Optical Properties**

Transparency Range | 160-2600nm |

SHG Phase Matchable Range | 551-2600nm (Type I) 790-2150nm (Type II) |

Therm-optic Coefficient (/℃, λ in μm) | dn_{x}/dT=-9.3X10^{-6
}dn_{y}/dT=-13.6X10^{-6
}dn_{z}/dT=(-6.3-2.1λ)X10^{-6} |

Absorption Coefficients | <0.1%/cm at 1064nm <0.3%/cm at 532nm |

Angle Acceptance | 6.54mrad·cm (φ, Type I,1064 SHG) 15.27mrad·cm (θ, Type II,1064 SHG) |

Temperature Acceptance | 4.7℃·cm (Type I, 1064 SHG) 7.5℃·cm (Type II, 1064 SHG) |

Spectral Acceptance | 1.0nm·cm (Type I, 1064 SHG) 1.3nm·cm (Type II, 1064 SHG) |

Walk-off Angle | 0.60° (Type I 1064 SHG) 0.12° (Type II 1064 SHG) |

NLO Coefficients | d_{eff}(I)=d_{32}cosΦ (Type I in XY plane)d _{eff}(I)=d_{31}cos^{2}θ+d_{32}sin^{2}θ (Type I in XZ plane)d _{eff}(II)=d_{31}cosθ (Type II in YZ plane)d _{eff}(II)=d_{31}cos^{2}θ+d_{32}sin^{2}θ (Type II in XZ plane) |

Non-vanished NLO susceptibilities | d_{31}=1.05± 0.09 pm/Vd _{32}= -0.98± 0.09 pm/Vd _{33}=0.05± 0.006 pm/V |

Sellmeier Equations (λ in μm) |
n_{x}^{2}=2.454140+0.011249/(λ^{2}-0.011350)-0.014591λ^{2}-6.60×10^{-5}λ^{4}n _{y}^{2}=2.539070+0.012711/(λ^{2}-0.012523)-0.018540λ^{2}+2.00×10^{-5}λ^{4}n _{z}^{2}=2.586179+0.013099/(λ^{2}-0.011893)-0.017968λ^{2}-2.26×10^{-5}λ^{4} |

**SHG and THG at Room Temperature**

LBO is phase matchable for the SHG and THG of Nd:YAG and Nd:YLF lasers, using either type I or type II interaction. For the SHG at room temperature, type I phase matching can be reached and has the maximum effective SHG coefficient in the principal XY and XZ planes in a wide wavelength range from 551nm to about 2600nm.

The optimum type II phase matching falls in the principal YZ and XZ planes.

SHG conversion efficiencies of more than 70% for pulse and 30% for cw Nd:YAG lasers, and THG conversion efficiency over 60% for pulse Nd:YAG laser have been observed by using Photonchina’s LBO crystals.

**Applications**

• More than 480mW output at 395nm is generated by frequency doubling a 2W mode-locked Ti:Sapphire laser (<2ps, 82MHz). The wavelength range of 700-900nm is covered by a 5x3x8mm^{3} LBO crystal.

• Over 80W green output is obtained by SHG of a Q-switched Nd:YAG laser in a type II 18mm long LBO crystal.

• The frequency doubling of a diode pumped Nd:YLF laser (>500μJ @ 1047nm,<7ns, 0-10KHz) reaches over 40% conversion efficiency in a 9mm long LBO crystal.

• The VUV output at 187.7 nm is obtained by sum-frequency generation.

• 2mJ/pulse diffraction-limited beam at 355nm is obtained by intracavity frequency tripling a Q-switched Nd:YAG laser.

**Non-Critical Phase Matching**

As shown in Table1, Non-Critical Phase Matching (NCPM) of LBO is featured by no walk-off, very wide acceptance angle and maximum effective coefficient. It promotes LBO to work in its optimal condition. SHG conversion efficiencies of more than 70% for pulse and 30% for cw Nd:YAG lasers have been obtained, with good output stability and beam quality.

As shown in Fig1, type I and type II non-critical phase matching can be reached along x-axis and z-axis at room temperature, respectively.(Photonchina develops an assembly of oven and temperature controller for NCPM applications.

Figure1. NCPM temperature tuning curves of LBO

Table1. Properties of type I NCPM SHG at 1064nm | |

NCPM Temperature Acceptance Angle Walk-off Angle Temperature Bandwidth Effective SHG Coefficient |
148℃ 52 mrad·cm 0 4℃·cm 2.69 x d36(KDP) |

** **

**Applications**

• Over 11W of average power at 532nm was obtained by extra-cavity SHG of a 25W Antares mode-locked Nd:YAG laser (76MHz, 80ps).

• 20W green output was generated by frequency doubling a medical, multi-mode Q-switched Nd:YAG laser. Higher green output is expected with higher input power.

**LBO’s OPO and OPA:**

** **** ** LBO is an excellent NLO crystal for OPOs and OPAs with a widely tunable wavelength range and high powers. These OPO and OPA which are pumped by the SHG and THG of Nd:YAG laser and XeCl excimer laser at 308nm have been reported. The unique properties of type I and type II phase matching as well as the NCPM leave a large room in the research and applications of LBO’s OPO and OPA.

**LBO’s Spectral NCPM:**

Not only the ordinary non-critical phase matching (NCPM) for angular variation but also the non-critical phase matching for spectral variation (SNCPM) can be achieved in the LBO crystal.The phase matching retracing positions are λ_{1}=1.31μm with θ =86.4°, φ=0° for Type I and λ_{2}=1.30 μm with θ =4.8°, φ=0° for Type II. The phase matching at these positions possesses very large spectral acceptances Δλ. The calculated Δλ at λ_{1} and λ_{2} are 57nm·cm and 74nm·cm respectively, which are much larger than that of other NLO crystals. These spectral characteristics are very suitable for doubling broadband coherent radiations near 1.3 μm, such as those from some diode lasers, and some OPA/OPO output without linewidth-narrowing components.

**AR-coatings**

Photonchina provides the following AR-coatings:

• Dual Band AR-coating (DBAR) of LBO for SHG of 1064nm.

• low reflectance (R<0.2% at 1064nm and R<0.5% at 532nm), super low reflectivity of R<0.05% at 1064nm and R<0.1% at 532nm is available upon request; high damage threshold (＞500MW/cm^{2}at both wavelengths); long durability.

• Broad Band AR-coating (BBAR) of LBO for SHG of tunable lasers.

• Other coatings are available upon request.

**Photonchina LBO Specifications**

- Dimension tolerance: (W±0.1mm)x(H±0.1mm)x(L+0.5/-0.1mm) (L≥2.5mm)

(W±0.1mm)x(H±0.1mm)x(L+0.1/-0.1mm) (L<2.5mm)

• Clear aperture: central 90% of the diameter

• No visible scattering paths or centers when inspected by a 50mW green laser

• Flatness: less than λ/8 @ 633nm

• Transmitting wavefront distortion: less than λ/8 @ 633nm

• Chamfer: ≤0.2mm x 45°

• Chip: ≤0.1mm

• Scratch/Dig code: better than 10/ 5 to MIL-PRF-13830B

• Parallelism: better than 20 arc seconds

• Perpendicularity: ≤5 arc minutes

• Angle tolerance: △θ≤0.25°, △φ≤0.25°

• Damage threshold[GW/cm^{2}]: >10 for 1064nm, TEM00, 10ns, 10HZ (polished only)

>1 for 1064nm, TEM00, 10ns, 10HZ (AR-coated)

>0.5 for 532nm, TEM00, 10ns, 10HZ (AR-coated)

• Quality Warranty Period: one year under proper use.

**Remarks**

- LBO has a low susceptibility to moisture. Users are advised to provide dry conditions for both the use and preservation of LBO.
- Polished surfaces of LBO requires precautions to prevent any damage.
- Photonchina engineers can select and design the best crystal for you, based on the main parameters of your laser, such as energy per pulse, pulse width and repetition rate for a pulsed laser, power for a cw laser, laser beam diameter, mode condition, divergence, wavelength tuning range, etc.
- For thin crystals, Photonchina can provide free holders for you.